1. Field of the Invention
The present invention relates to a ladder-type piezoelectric filter including a series resonator and a parallel resonator which are connected so as to define a ladder-type circuit.
2. Description of the Related Art
In a conventional ladder-type piezoelectric filter, square-shaped piezoelectric elements adapted to vibrate in a square type vibration mode are used as a series resonator and a parallel resonator. The resonators are stacked in layers in a thickness direction with terminal plates disposed therebetween.
As shown in FIG. 1, the ladder-type filter obtains a desired filter characteristic by utilizing the impedance characteristic of the parallel resonator at the resonance frequency Fr and the impedance characteristic of the series resonator at the antiresonance frequency Fa.
For example, in order to obtain a filter having a center frequency of 455 kHz (xcex94f=20), a series resonator having a Fr of 455 kHz and a parallel resonator having a Fr of 435 kHz are produced. If the series resonator and the parallel resonator are made from a material having a frequency constant of 2000 kHzxc2x7mm, the series and parallel resonators have the following dimensions:
Series resonator: 2000÷455=4.396 mm
Parallel resonator: 2000÷435=4.598 mm
Thus, a dimensional difference of about 0.2 mm occurs between the series resonator and the parallel resonator.
Since the series and parallel resonators are contained within the same housing, the housing is made so as to accommodate the larger parallel resonator, resulting in a displacement between the center of the parallel resonator and the center of the series resonator.
As described above, when series and parallel resonators having a square shape are made from materials having the same frequency constant, the series and parallel resonators have different sizes. As a result, when the series and parallel resonators are housed, their respective centers do not align with each other. A square-shaped resonator adapted to vibrate in a square type vibration mode must be held at the center thereof by a terminal plate. Otherwise, proper vibration is hindered and a filter characteristic curve is distorted.
A square-shaped resonator adapted to vibrate in a square type vibration mode causes a main impedance variation at Fa and Fr and a spurious impedance variation at a higher frequency. When square resonators of a square type vibration mode are used as the series and parallel resonators, the spurious impedance variations are superposed on each other, resulting in an undesirable filter characteristic.
To overcome the above described problems, one of the preferred embodiments of the present invention provides a ladder-type piezoelectric filter including a first piezoelectric element adapted to vibrate in a square type vibration mode having a substantially square shape and functioning as a parallel resonator, and a second piezoelectric element which functions as a series resonator and is adapted to vibrate in a square type vibration mode, the second piezoelectric element being made from a material having a frequency constant that is substantially equal to that of a material from which the first element is made, the second piezoelectric element having a substantially square shape such that a distance between opposite side edges thereof is substantially equal to that of the first piezoelectric element and such that each of four corners of the second piezoelectric element is cut.
When a substantially square-shaped series resonator and a substantially square parallel resonator are made from materials having the same frequency constant, the resulting series and parallel resonators differ in size. However, when a substantially square-shaped resonator is used as a parallel resonator and a substantially square-shaped resonator having corner cuts is used as a series resonator as described above, the resonance frequency of the parallel resonator differs from that of the series resonator even though the parallel and series resonators have substantially equal distances between respective opposite side edges thereof, so that a desired ladder-type filter is achieved.
Another preferred embodiment of the present invention provides a ladder-type piezoelectric filter including a first piezoelectric element adapted to vibrate in a square type vibration mode having a substantially square shape and functioning as a parallel resonator, and a second piezoelectric element functioning as a series resonator, adapted to vibrate in a square type vibration mode and made from a material having a frequency constant that is substantially equal to that of a material from which the first piezoelectric element is made, the second piezoelectric element having a substantially square shape such that a distance between opposite side edges thereof is substantially equal to that of the first piezoelectric element and such that each nodal portion located at an approximate central portion of each of the side edges is cut.
A further preferred embodiment of the present invention provides a ladder-type piezoelectric filter including a first piezoelectric element adapted to vibrate in a square type vibration mode, having a substantially square shape such that each of four corners thereof is cut, and functioning as a series resonator, and a second piezoelectric element functioning as a parallel resonator, adapted to vibrate in a square type vibration mode, being made from a material having a frequency constant that is substantially equal to that of a material from which the first piezoelectric element is made, the second piezoelectric element having a substantially square shape such that a distance between opposite side edges thereof is substantially equal to that of the first piezoelectric element and such that each of a nodal portion located at an approximate central portion of each of side edges thereof is cut.
In such a ladder-type filter including resonators of two different kinds which have substantially equal distances between respective opposite side edges thereof, the parallel and series resonators can be housed in a center-aligned arrangement, thereby preventing hindrance to proper vibration and distortion of a filter characteristic curve.
In the case of a square resonator of a square type vibration mode, a spurious impedance variation appears at a frequency higher than that of a main impedance variation. In the case of a resonator having nodal cuts, a spurious impedance variation shifts toward the higher-frequency side as compared with the case of the square resonator. In the case of a resonator having corner cuts, a spurious impedance variation shifts toward the lower-frequency side as compared with the case of the square resonator. Through a combination of series and parallel resonators of different shapes as in the cases of the preferred embodiments described above, superposition of the respective spurious impedance variations can be prevented, thereby providing a greatly improved characteristic in relation to spurious impedance variation.
An additional preferred embodiment of the present invention provides a ladder-type piezoelectric filter including a first piezoelectric element adapted to vibrate in a square type vibration mode, having a substantially square shape such that each of four corners thereof is cut, and functioning as a series resonator, and a second piezoelectric element functioning as a parallel resonator, adapted to vibrate in a square type vibration mode, being made from a material having a frequency constant that is substantially equal to that of a material from which the first piezoelectric element is made, the second piezoelectric element having a substantially square shape such that a distance between opposite side edges thereof is substantially equal to that of the first piezoelectric element and such that each of four corners thereof is cut in which each of the cut portions of the second piezoelectric element is narrower than each of the cut portions of the first piezoelectric element.
As in the case of the above described ladder-type piezoelectric filter, the parallel and series resonators can be housed in a center-aligned arrangement, and superposition of the respective spurious impedance variations can be prevented. Since the element having wide corner cuts is higher in resonant frequency than the element having narrow corner cuts, the element having wide corner cuts is used as the series resonator, and the element having narrow corner cuts is used as the parallel resonator.
An additional preferred embodiment of the present invention provides a ladder-type piezoelectric filter including a first piezoelectric element adapted to vibrate in a square type vibration mode, having a substantially square shape such that each of a nodal portion located at an approximate central portion of each of side edges thereof is cut, and functioning as a series resonator, and a second piezoelectric element functioning as a parallel resonator and adapted to vibrate in a square type vibration mode, being made from a material having a frequency constant which is substantially equal to that of a material from which the first piezoelectric element is made, having a substantially square shape such that a distance between opposite side edges is substantially equal to that of the first piezoelectric element and such that each of a nodal portion located at an approximate central portion of each of side edges is cut deeper than that of the first piezoelectric element.
The above-described ladder-type piezoelectric filter achieves the advantages and effects similar to those of the ladder-type filter of the first preferred embodiment described above.
Since the element having shallow cuts is higher in resonant frequency than the element having deep cuts, the element having shallow cuts is used as the series resonator, and the element having deep cuts is used as the parallel resonator.
Other features and advantages of the present invention will become apparent from the following description of preferred embodiments of the present invention which refers to the accompanying drawings.